Update: Please post new comments in the latest HPMOR discussion thread, now in the discussion section, since this thread and its first few successors have grown unwieldy (direct links: two, three, four, five, six, seven).
As many of you already know, Eliezer Yudkowsky is writing a Harry Potter fanfic, Harry Potter and the Methods of Rationality, starring a rationalist Harry Potter with ambitions to transform the world by bringing the rationalist/scientific method to magic. But of course a more powerful Potter requires a more challenging wizarding world, and ... well, you can see for yourself how that plays out.
This thread is for discussion of anything related to the story, including insights, confusions, questions, speculation, jokes, discussion of rationality issues raised in the story, attempts at fanfic spinoffs, comments about related fanfictions, and meta-discussion about the fact that Eliezer Yudkowsky is writing Harry Potter fan-fiction (presumably as a means of raising the sanity waterline).
I'm making this a top-level post to create a centralized location for that discussion, since I'm guessing people have things to say (I know I do) and there isn't a great place to put them. fanfiction.net has a different set of users (plus no threading or karma), the main discussion here has been in an old open thread which has petered out and is already near the unwieldy size that would call for a top-level post, and we've had discussions come up in a few other places. So let's have that discussion here.
Comments here will obviously be full of spoilers, and I don't think it makes sense to rot13 the whole thread, so consider this a spoiler warning: this thread contains unrot13'd spoilers for Harry Potter and the Methods of Rationality up to the current chapter and for the original Harry Potter series. Please continue to use rot13 for spoilers to other works of fiction, or if you have insider knowledge of future chapters of Harry Potter and the Methods of Rationality.
A suggestion: mention at the top of your comment which chapter you're commenting on, or what chapter you're up to, so that people can understand the context of your comment even after more chapters have been posted. This can also help people avoid reading spoilers for a new chapter before they realize that there is a new chapter.
It seems like the spells in the HP universe are complicated and abstract enough that they must have been designed (programmed?) by wizards long ago, who added them to the laws of the universe and left them there.
Now, if I were designing a spell like the Killing Curse, I would include a little easter egg/safety mechanism: after a thousand castings, it backfires. Choose a number large enough that only a major dark wizard like Voldemort will encounter it, so it doesn't hit some minor villain and spoil the surprise. (Alternatively, rather than counting kills, count evilness, with killing a baby counting for more evilness points than an adult. That would explain why it backfired on Harry Potter, rather than some other victim.)
This is the most sensible explanation I can come up with. Or it could be that it backfired because the third through fifteenth places of the decimal expansion of the local humidity were a prime number, or something similarly arbitrary. But I would be disappointed if it was something like that. (I would also be disappointed if his parents came up with a spell that reflected it, because everyone seems convinced that no such spell is possible.)
Or maybe it just doesn't work on children? No one knows because no one's ever tried it.
If you could program a slaying weapon, what is the one group of people that no-one in their right mind could possibly ever want to kill? I'd say that group would be children too young to speak. Anyone going after them is certainly an absolute psycho.